1. Field of the Invention
This invention relates to the detection of gasses at long distance and more particularly the detection of hazardous or chemical weapon gasses by detection of optical response.
2. Description of the Related Art
Chemical warfare agents (CWAs), explosives vapors and many toxic industrial chemicals (TICs) absorb radiation in the medium wave infrared (MWIR) and long wave infrared (LWIR) region. This is the basis of high sensitivity, high selectivity low probability of false alarm (PFA) detection of these targets using to make MWIR and LWIR laser photoacoustic spectroscopy (L-PAS). L-PAS is a point sensor system. The method and technology of using tunable IR lasers, photoacoustic spectroscopy sensitivity and calculation of receiver operational characteristic (ROC) curve have been described in a number of papers and/or publications by the inventors and their colleagues at Pranalytica, Inc. of Santa Monica, Calif. (incorporated herein by reference).
For single-ended standoff detection applications, traditional light detection and ranging (LIDAR) and differential absorption LIDAR (DIAL) measurement schemes are less than satisfactory because of the small Raman scattering signal returns (for LIDAR) and small Rayleigh scattering returns (for DIAL) from molecular clouds containing explosives vapors and CWAs, especially when the target gasses are present at low concentrations. On the other hand, for a CWA gas cloud containing explosives vapors or CWAs even at ppm concentrations, the infrared absorption coefficients are large enough (in the range of 10−2 to 10−3 m−1 at 1 ppm concentration) to detect their presence, if the cloud dimensions are reasonably large (tens to hundreds of meters), as would be the case, for example, for intentional release of a chemical warfare agent. Table 1 gives typical numbers for Raman scattering, Rayleigh scattering and infrared absorption cross-sections. An initial choice of 1 ppm target may be selected from the consideration of human sensitivity to Sarin exposure of which varies from 17 ppm for lethality [1] to 170 ppb for miosis [2] for a one minute exposure. Furthermore, for a standoff detection of triacetone triperoxide (TATP), 1 ppm of vapor pressure corresponds to a TATP temperature of −5° C. [3,4].
TABLE 1Typical values for Raman scattering, Rayleigh scattering and infraredabsorption cross-sections (note that numbers may vary by 1-2 orders of magnitude depending on the specific molecular species)ProcessCross-SectionRaman scattering~10−30 cm2 molecule−1Rayleigh scattering~10−28 cm2 molecule−1Infrared absorption~10−17 cm2 molecule−1
Infrared absorption measurements are the basis of the L-PAS sensors that are able to detect CWAs at ppb levels in a path length of 10 cm (point sensor). Point detection of TATP at vapor pressures down to about 1 ppb corresponding to a TATP temperature of −45° C. using laser photoacoustic spectroscopy is known [4]. Thus, measuring absorption rather than scattering appears to be more important for standoff detection. Of course, if a double-ended system were to be acceptable, standoff detection of tens of meters of a CWA cloud would be straightforward. However, for a single-ended detection scheme, the problem is significantly harder and provides significant additional obstacles.
Known CWAs include: Cyclosarin (GF), Sarin (GB), Soman (GD), Tabun (GA), VX, some insecticides, Novichok agents, most arsines, cyanogen chloride, hydrogen cyanide, sulfur mustard (HD, H), nitrogen mustard (HN-1, HN-2, HN-3), Lewisite (L), Phosgene oxime (CX), chlorine, hydrogen chloride, nitrogen oxides, phosgene, tear gas, pepper spray, Agent 15 (BZ), and non-living biological proteins such as ricin and abrin. The foregoing list is not complete, but gives an indication of the types of chemicals that are used as chemical warfare agents (CWAs). Similar lists are known for explosives and/or explosives vapors as well as toxic industrial chemicals (TICs).
One such list for TICs is available through OSHA (the U.S. Department of Labor's Occupational Safety & Health Administration). OSHA maintains a web site listing TICs at the URL http://www.osha.gov/SLTC/emergencypreparedness/guides/chemical.html which indicates as follows:
Toxic industrial chemicals are industrial chemicals that are manufactured, stored, transported, and used throughout the world. Toxic industrial chemicals can be in the gas, liquid, or solid state. They can be chemical hazards (e.g., carcinogens, reproductive hazards, corrosives, or agents that affect the lungs or blood) or physical hazards (e.g., flammable, combustible, explosive, or reactive). The following table lists the most common TICs listed by their hazard index.
TICs listed by hazard indexHighMediumLowAmmoniaAcetone cyanohydrinAllyl isothiocyanate(CAS# 7664-41-7)(CAS# 75-86-5)(CAS# 57-06-7)ArsineAcroleinArsenic trichloride(CAS# 7784-42-1).(CAS# 107-02-8)(CAS# 7784-34-1)Boron trichlorideAcrylonitrileBromine(CAS# 10294-34-5)(CAS# 107-13-1)(CAS# 7726-95-6)Boron trifluorideAllyl alcoholBromine chloride(CAS# 7637-07-2)(CAS# 107-18-6)(CAS# 13863-41-7)Carbon disulfideAllylamineBromine pentafluoride(CAS# 75-15-0)(CAS# 107-11-9)(CAS# 7789-30-2)ChlorineAllyl chlorocarbonateBromine trifluoride(CAS# 7782-50-5)(CAS# 2937-50-0)(CAS# 7787-71-5)DiboraneBoron tribromideCarbonyl fluoride(CAS# 19287-45-7)(CAS# 10294-33-4)(CAS# 353-50-4)Ethylene oxideCarbon monoxideChlorine pentafluoride(CAS# 75-21-8)(CAS# 630-08-0)(CAS# 13637-63-3)FluorineCarbonyl sulfideChlorine trifluoride(CAS# 7782-41-4)(CAS# 463-58-1)(CAS# 7790-91-2)FormaldehydeChloroacetoneChloroacetaldehyde(CAS# 50-00-0)(CAS# 78-95-5)(CAS# 107-20-0)Hydrogen bromideChloroacetonitrileChloroacetyl chloride(CAS# 10035-10-6)(CAS# 7790-94-5)(CAS# 79-04-9)Hydrogen chlorideChlorosulfonic acidCrotonaldehyde(CAS# 7647-01-0)(CAS# 7790-94-5)(CAS# 123-73-9)Hydrogen cyanideDiketeneCyanogen chloride(CAS# 74-90-8)(CAS# 674-82-8)(CAS# 506-77-4)Hydrogen fluoride1,2-DimethylhydrazineDimethyl sulfate(CAS# 7664-39-3)(CAS# 540-73-8)(CAS# 77-78-1)Hydrogen sulfideEthylene dibromideDiphenylmethane-4.4′-(CAS# 7783-0604)(CAS# 106-93-4)diisocyanate(CAS# 101-68-8)Nitric acid, fumingHydrogen selenideEthyl chlroroformate(CAS# 7697-37-2)(CAS# 7783-07-5)(CAS# 541-41-3)PhosgeneMethanesulfonyl chlorideEthyl chlorothioformate(CAS# 75-44-5)(CAS# 124-63-0)(CAS# 2941-64-2)Phosphorus trichlorideMethyl bromideEthyl phosphonothioic(CAS# 7719-12-2)(CAS# 74-83-9)dichloride(CAS# 993-43-1)Sulfur dioxideMethyl chloroformateEthyl phosphonic (CAS# 7446-09-5)(CAS# 79-22-1)dichloride(CAS# 1066-50-8)Sulfuric acidMethyl chlorosilaneEthyleneimine(CAS# 7664-93-9)(CAS# 993-00-0)(CAS# 151-56-4)Tungsten hexafluorideMethyl hydrazineHexachlorocyclo-(CAS# 7783-82-6)(CAS# 60-34-4)pentadiene(CAS# 77-47-4)Methyl isocyanateHydrogen iodide(CAS# 624-83-9)(CAS# 10034-85-2)Methyl mercaptanIron pentacarbonyl(CAS# 74-93-1)(CAS# 13463-40-6)Nitrogen dioxideIsobutyl chloroformate(CAS# 10102-44-0)(CAS# 543-27-1)PhosphineIsopropyl (CAS# 7803-51-2)chloroformate(CAS# 108-23-6)Phosphorus oxychlorideIsopropyl isocyanate(CAS# 10025-87-3)(CAS# 1795-48-8)Phosphorus pentafluoriden-Butyl chloroformate(CAS# 7647-19-0)(CAS# 592-34-7)Selenium hexafluoriden-Butyl isocyanate(CAS# 7783-79-1)(CAS# 111-36-4)Silicon tetrafluorideNitric oxide(CAS# 7783-61-1)(CAS# 10102-43-9)Stibinen-Propyl (CAS# 7803-52-3)chloroformate(CAS# 109-61-5)Sulfur trioxideParathion(CAS# 7446-11-9)(CAS#: 56-38-2)Sulfuryl fluoridePerchloromethyl (CAS# 2699-79-8)mercaptan(CAS# 594-42-3)Tellurium hexafluoridesec-Butyl (CAS# 7783-80-4)chloroformate(CAS# 17462-58-7)n-Octyl mercaptantert-Butyl isocyanate(CAS# 111-88-6)(CAS# 1609-86-5)Titanium tetrachlorideTetraethyl lead(CAS# 7550-45-0)(CAS# 78-00-2)Tricholoroacetyl chlorideTetraethyl (CAS# 76-02-8)pyroposphate(CAS# 107-49-3)Trifluoroacetyl chlorideTetramethyl lead(CAS# 354-32-5)(CAS# 75-74-1)Toluene 2.4-diisocyanate(CAS# 584-84-9)Toluene 2.6-diisocyanate(CAS# 91-08-7)
Some of the foregoing OSHA-listed TICs may be either gasses or subject to aerosolization.